Rubber mulch production is one commercial solution to the problem of disposing of millions of used tires discarded each year. Available in several natural colors, rubber mulch is heavier than wood mulches. Slow to break down, it eliminates annual mulching. A rubber mulch ground cover provides a soft landing surface where children play. Rubber mulch, however, has its disadvantages.
Rubber mulch, shaped to look like shredded wood, comes in a variety of natural wood and designer colors. Dark brown, redwood and cypress are typical wood colors, while designer colors include black, purple, green and blue. Some manufacturers provide custom dyes so that homeowners can coordinate their home and landscape colors.
Consumer Reports website performed a 2008 comparison test pitting two brands of rubber mulch against one of wood. Each was set in its own partially covered tray. The trays were placed in an exposed location for 32 weeks. At the end of the test, the wood mulch and one of the rubber mulches had slight fading, while the other rubber mulch was unchanged. Claims that rubber mulch ground covers retain their color for years may not be accurate.
The Consumer Reports tests included flooding and using a leaf blower near all three mulches. While the wood mulch either floated or blew away, the rubber mulches proved more stable. Neither of them floated. The leaf blower, standing between 24 and 36 inches away and set on high, disturbed one of the mulches slightly and the other not at all.
Consumer Reports and the Washington State University Extension agree that rubber mulch ground cover is a greater fire hazard than wood. While none of the three tested mulches ignited from a lit cigarette, all did from a lit match. The wood mulch fire, however, extinguished much more easily than the rubber mulch ones. WSU's professor Linda Chalker-Scott advises against using a rubber mulch ground cover in locations with elevated fire risks.
Rubber Mulch and Zinc
Plants in soil with high zinc levels grow at a reduced rate, according to North Carolina Department of Agriculture agronomist M. Ray Tucker. The tires from which rubber mulch comes contain more than 1 1/2 percent zinc. The zinc can leach into acidic soil, where plants absorb it. Sand mixed with as little as 2 percent ground rubber can cause zinc toxicity. Symptoms include wilting, discolored foliage.
Vegetables in soil that measures high in the heavy metal cadmium, however, will benefit from the zinc. Some vegetables, including spinach, accumulate cadmium in toxic amounts. Soils also rich in zinc, however, prevent high levels of cadmium accumulation. Using rubber mulch as a ground cover for these crops is a less expensive way of raising soil zinc levels than using fertilizers.